Method and device for tapping undersea freshwater

ABSTRACT

The invention relates to a device for tapping at least one undersea freshwater spring that comprises: an housing ( 110 ) surrounded by salt water and isolating an amount of water above at least one freshwater spring; a sensor ( 150, 152, 155 ) for sensing the depth of an interface ( 170 ) between the freshwater and the salt water inside said housing; and a means ( 145 ) for controlling said depth and adapted for controlling the flow rate of at least one pump ( 125 ) for pumping freshwater above said interface depending on said depth, wherein said control means is adapted for maintaining the depth of said interface underneath the outlet of at least one freshwater spring.

This invention concerns a process and a device for the collection ofundersea freshwater. It concerns, in particular, the collection offreshwater derived from undersea freshwater resurgences coming fromkarst networks.

The karst is a domain of calcareous rocks comprising manyinter-communicating cavities, created by the calcium carbonate beingdissolved by carbonic acid derived from rainwater loaded with carbondioxide (CO₂). All these cavities make up a karst network, the lowerpart of which constitutes significant reservoirs of freshwater.

For reasons connected to geological evolution, the karsts, especiallythose of the Mediterranean rim, have often been lowered by several tensof meters, or even hundreds of meters in places, and as a result theirbase level can now be located below sea level. It is known thatfreshwater springs coming from karst domains flow under the sea.Freshwater, with a lower density than seawater, rises towards thesurface of the sea.

Attempts have been made to collect the freshwater from undersea karstresurgences, based on the use of a rigid barrier.

French patent no. FR 2 701 974 describes a submerged construction,comprising a concave part aimed downwards and covering the spring, thefreshwater remaining trapped in the upper part of said cavity thanks toits density, which is less than that of the seawater. The lower part ofthis construction remains open so as to let the overflow, and inparticular the seawater, escape, at the same time leaving the karstsystem unrestricted and removing the freshwater from the upper part ofthe construction by means of a variable-flow pumping system. The pumpingdevice is subordinate to sensors, which enable a flow rate slightly lessthan the spring's flow rate to be maintained. This device allowsfreshwater to be removed without the existing equilibrium beingdisturbed and without introducing disruptions into the working of thekarst system, facilitating the laminar flow, which obviates the mixingwith seawater.

Document FR 2 785 001 describes a device whose purpose is an improvementenabling a significant simplification of the collection system byutilizing an impermeable surface, for preference flexible, whichisolates the resurgence, anchored to the sea bottom and reaching thesurface where it is held by a floating barrier, allowing the ventilationof the collection device. The device also comprises a system of valvesin the lower part of the membrane, allowing the seawater to escape; thefreshwater is removed by pumps installed on a floating device on theinside surface of the barrier.

Document WO 2007/017703 proposes to add, to the device of document FR 2785 001, a current meter for each spring in order to control the pumpsand thus maintain an equilibrium in the system. However, due to theexistence of a large number of unpredictable springs and infiltrations,the current meters only give an imprecise measurement of the flow-ratesavailable and the efficiency of this system is limited.

Document FR 2 795 109 proposes a flexible envelop forming a dome over anundersea freshwater spring. A salinity sensor is provided so that thepump opening draws in freshwater. However, as the spring of freshwater,which is lighter, is below the seawater, which is heavier, these twotypes of water mix. The water thus pumped therefore requires adesalination treatment practically as complex and expensive as theseawater desalination treatment.

The aim of the present invention is to respond to these drawbacks.

To this end, according to a first aspect, the present inventionenvisages a device for collecting water from at least one underseafreshwater spring, characterized in that it comprises:

-   -   an envelop surrounded by seawater isolating a quantity of water        above at least one freshwater spring,    -   a sensor of the status of an interface between the freshwater        and the seawater, inside said envelop and    -   a means of closed-loop control designed to control the flow-rate        of at least one pump designed to pump freshwater above said        interface, according to said status of said interface, said        means of closed-loop control being designed to maintain the        depth of said interface below the opening of at least one said        freshwater spring.

Thanks to these provisions, according to the status of the interface,for example its depth or the turbulences causing a freshwater andseawater mixture there, more or less water is pumped. For example, themore the interface is subjected to turbulences, the more the interface'sdepth is increased, so that there is no risk of the freshwater comingfrom the springs being mixed with seawater as a result of thisturbulence. In addition, apart from transitional phases during which themovement of the interface is commanded, the quantity of water pumpedrepresents the quantity of freshwater coming from these springs,whatever the number, flow-rate, configuration of the springs and/orinfiltrations.

In addition, as the freshwater coming from each of the freshwater springopenings is above the interface, there is little risk of the freshwaterbeing mixed with seawater.

According to particular features, the sensor of the status of saidinterface comprises a depth sensor designed to determine the depth ofsaid interface and the means of closed-loop control is designed tocontrol the depth of said interface by controlling the flow-rate of atleast one pump according to said depth.

According to particular features, the sensor of the status of saidinterface comprises a means of determining the agitation of saidinterface and the means of closed-loop control is designed to controlthe flow-rate of at least one pump according to said agitation.

According to particular features, the means of closed-loop control isdesigned to reduce the flow-rate of at least one said pump when saidinterface presents turbulences greater than a pre-defined value.

Thus, when the sea is agitated or the flow-rate of the springs increasesto the point of causing turbulences, you can stop pumping freshwater inorder to avoid pumping brackish water or spring water bearing earth orother impurities, or reduce the rate pumped so that the interface'sdepth is increased and is farther from the opening of each freshwaterspring.

Other particular features of this first aspect of the present inventionare presented following the third and fifth aspects of the presentinvention.

According to a second aspect, the present invention envisages a processfor collecting water from at least one undersea freshwater spring,characterized in that it comprises:

-   -   a step of acquiring the status of an interface between the        freshwater and the seawater, inside an envelop surrounded by        seawater isolating a quantity of water above at least one        freshwater spring and    -   a step of closed-loop control during which the flow-rate of at        least one pump pumping freshwater above said interface is        controlled according to said status of said interface, in order        to maintain the depth of said interface below the opening of at        least one said freshwater spring.

As the advantages, aims and special features of this process that is thesubject of the second aspect of the present invention are similar tothose of the device that is the subject of first aspect of the presentinvention, as described in brief above, they are not repeated here.

According to a third aspect, the present invention envisages a devicefor collecting water from at least one undersea freshwater spring,characterized in that it comprises:

-   -   an envelop surrounded by seawater isolating a quantity of water        above at least one freshwater spring,    -   a sensor of the depth of an interface between the freshwater and        the seawater, inside said envelop and    -   a means of closed-loop control of this depth designed to control        the flow-rate of at least one pump designed to pump freshwater        above said interface, according to said depth.

Thanks to these provisions, the quantity of water pumped represents thequantity of freshwater coming from these springs, whatever the number,flow-rate, configuration of the springs and/or infiltrations.

According to particular features, the depth sensor comprises a sensor ofthe difference in level between the surface of the freshwater inside theenvelop collecting the freshwater and the surface of the seawater aroundsaid envelop.

Thanks to these provisions, the measurement can be accurate andmaintenance easy.

According to particular features, the depth sensor comprises anequilibrium depth sensor for a float possessing a density half-waybetween the density of the freshwater and the density of the seawater.

Thanks to these provisions, the measurement can be especially stable andaccurate.

According to particular features, the means of closed-loop control isdesigned to maintain said interface below the level of at least onespring.

According to particular features, the means of closed-loop control isdesigned to maintain said interface below the level of at least thespring with the greatest flow-rate.

According to particular features, the means of closed-loop control isdesigned to control the flow-rate of at least one pump according to thesea level during the tide.

Thanks to these provisions, the interface's absolute vertical positioncan remain noticeably stable even though the depth below the immediatelevel of the water inside and/or outside the envelop depends on thetidal phenomenon and varies cyclically.

According to particular features, said envelop comprises a flexiblepartition surrounding the vertical above at least one spring.

According to particular features, said flexible partition is kept inposition by weights.

According to particular features, said flexible partition comprises, inits upper part, inflated booms.

According to particular features, the device as described in brief abovecomprises a means of determining the agitation of the water outside atleast one source of water, the means of closed-loop control beingdesigned to control the flow-rate of at least one pump according to theagitation of the water.

In this way it is possible to avoid pumping freshwater that has been toomuch mixed with seawater on output from the spring, because of the localagitation of the water.

According to particular features, the means of determining the agitationof the water comprises a radar or sonar doppler-effect sensor.

In this way the said agitation can be determined remotely withoutintroducing a physical sensor into the spring water flow and thus youcan avoid creating disturbances in this flow, which may lead tofreshwater and seawater being mixed.

According to particular features, the means of determining the agitationof the water comprises at least one water agitation sensor positioned inthe water and a means of measuring movements of each said sensor.

According to particular features, the means of determining the agitationof the water comprises a means of determining a density gradient of thewater.

According to particular features, the device as described in brief abovecomprises a means of measuring the salinity of the water pumped or to bepumped and a means of stopping pumping when said salinity is greaterthan a pre-defined value.

Other particular features of this third aspect of the present inventionare presented following the first and fifth aspects of the presentinvention.

According to a fourth aspect, the present invention envisages a processfor collecting water from at least one undersea freshwater spring,retained by an envelop surrounded by seawater isolating a quantity ofwater above at least one freshwater spring, characterized in that itcomprises:

-   -   a step of acquiring the depth of an interface between the        freshwater and the seawater, inside said envelop and    -   a closed-loop control step controlling this position in order to        control the flow-rate of at least one pump designed to pump        freshwater above said interface, according to said depth.

As the advantages, aims and special features of this process that is thesubject of the fourth aspect of the present invention are similar tothose of the device that is the subject of third aspect of the presentinvention, as described in brief above, they are not repeated here.

According to a fifth aspect, the present invention envisages a devicefor collecting water from at least one undersea freshwater spring,characterized in that it comprises:

-   -   an envelop surrounded by seawater isolating a quantity of water        above at least one freshwater spring,    -   a means of determining the agitation of the water outside at        least one source of water, inside said envelop and    -   a means of closed-loop control designed to control the flow-rate        of at least one pump designed to pump freshwater inside said        envelop, according to the agitation of the water.

In this way it is possible to avoid pumping freshwater that has been toomuch mixed with seawater on output from the spring, because of the localagitation of the water.

According to particular features, the means of determining the agitationof the water comprises a radar or sonar doppler-effect sensor.

In this way the said agitation can be determined remotely withoutintroducing a physical sensor into the spring water flow and thus youcan avoid creating disturbances in this flow, which may lead tofreshwater and seawater being mixed.

According to particular features, the means of determining the agitationof the water comprises at least one water agitation sensor positioned inthe water and a means of measuring movements of each said sensor.

According to particular features, the means of determining the agitationof the water comprises a means of determining a density gradient of thewater.

According to particular features, the means of determining the agitationof the water comprises a sensor of the difference in level between thesurface of the freshwater inside an envelop collecting the freshwaterand the surface of the seawater around said envelop.

Thanks to these provisions, the measurement can be accurate andmaintenance easy.

According to particular features, the means of determining the agitationof the water comprises an equilibrium depth sensor for a floatpossessing a density half-way between the density of the freshwater andthe density of the seawater.

Thanks to these provisions, the measurement can be especially stable andaccurate.

According to particular features, the means of closed-loop control isdesigned to control the flow-rate of at least one pump according to thesea level during the tide.

Thanks to these provisions, the interface's absolute vertical positioncan remain noticeably stable even though the depth below the immediatelevel of the water inside and/or outside the envelop depends on thetidal phenomenon and varies cyclically.

According to particular features, said envelop comprises a flexiblepartition surrounding the vertical above at least one spring.

According to particular features, said flexible partition is kept inposition by weights.

According to particular features, said flexible partition comprises, inits upper part, inflated booms.

Other particular features of this fifth aspect of the present inventionare presented following the first and third aspects of the presentinvention.

According to a sixth aspect, the present invention envisages a processfor collecting water from at least one undersea freshwater spring,retained by an envelop surrounded by seawater isolating a quantity ofwater above at least one freshwater spring, characterized in that itcomprises:

-   -   a step of determining the agitation of the water outside at        least one source of water, inside said envelop and    -   a means of controlling the flow-rate of at least one pump        designed to pump freshwater inside the envelop, according to the        agitation of the water.

As the advantages, aims and special features of this process that is thesubject of the sixth aspect of the present invention are similar tothose of the device that is the subject of fifth aspect of the presentinvention, as described in brief above, they are not repeated here.

Other advantages, aims and characteristics of this invention will becomeapparent from the description that will follow, made, as an example thatis in no way limiting, with reference to the drawings included in anappendix, in which:

FIG. 1 represents, schematically, in a top view, a particular embodimentof the device that is the subject of this invention,

FIG. 2 represents, schematically, in cross-section, the particularembodiment of the device that is the subject of this invention shown inFIG. 1 and

FIG. 3 represents, in the form of a logical diagram, steps in aparticular embodiment of the process that is the subject of thisinvention.

Throughout the description the term “float” relates to an element with adensity less than at least one of the liquids utilized, generally salineseawater, and which, as result, rises to the interface between thisliquid and a less-dense liquid, generally water that is less salty, orthe air.

FIGS. 1 and 2 show a fissure 100, natural or artificial, in whichundersea spring water resurgences 105 arrive, for example of karsticorigin. This fissure 100 is fitted with a flexible hermetic envelop 110linked to the side walls 115 of the fissure 100 in a hermetic way.Valves 120 are formed in the lower part of the flexible envelop 110 toallow the evacuation of any water overflow appearing inside the envelopwithout allowing, or by restricting, the entry of seawater inside theenvelop 110. These valves are, for example, formed from free portions ofthe flexible envelop 110, with dimensions greater than those of openingsformed in the envelop 110 opposite which these free portions arelocated, outside the envelop 110. Envelop 110 comprises a flexiblepartition surrounding the vertical above at least one spring.

At the immediate outlet of the freshwater from at least one spring, asystem of deflectors imposes a forced route on the flowing freshwater,in order to reduce turbulences and to induce a laminar flow offreshwater.

A pump 125, borne by a raft 130, is linked by pipes 135 to apurification unit 140. The pump 125 is controlled by a control unit 145linked to sensors 150, 151, 152 and 155. The flexible envelop 110 issurrounded, in its upper part, by compressed air booms 160 and isretained, in its lower part, by weights or lines attached to the oceanfloor and/or the edges of the fissure.

In a variant, the pump 125 is replaced by a plurality of pumps.

At least one sensor 150 is designed to measure the average difference oflevel between the surface of the water inside the flexible envelop 110and the surface of the water outside the flexible envelop 110. Thisdifference is averaged over a long period, compared to the wave periodon the sea surface, for example over two minutes, and represents, as aconsequence of buoyancy, the depth at which the interface 170 is foundbetween the seawater, which is more dense, and the spring water, whichis less dense. This sensor 150 is, for example, comprised of two levelsensors, one positioned close to the envelop 110 on the inner side, andthe other positioned close to the envelop 110 on the outer side.

At least one sensor 151 is designed to measure the salinity of the waterclose to the inlet or in the water circuit of the pump 125. Themeasurement is performed, for example, by measuring an electricalcharacteristic (resistance or capacitance) of a strip of water with aknown thickness, saline water being more conductive than pure water.

At least one sensor 152 is designed to measure the average level, underthe surface inside the envelop, at which water with a pre-defineddensity is found. This sensor is, for example, comprised of a radar, asonar or a “float” the density of which is half-way between the densityof the freshwater and the density of the seawater, “float” of which thedepth under the surface is measured. It can also be comprised of aDoppler-effect sensor measuring the speed of the interface based on thepartial reflection of sound waves on this interface.

At least one sensor 155 is designed to measure the agitation of thewater at the location of at least one freshwater resurgence. This sensor155 is, for example, comprised of a radar, a sonar or a plurality of“floats” the density of which is half-way between the density of thefreshwater and the density of the seawater, and possibly different,“floats” whose respective movements are measured. As is understood, thesensors 152 and 155 can be coupled in order to provide the twomeasurements in question.

The agitation sensor 155 can also be comprised of a turbulence sensor,for example acquiring the infrasounds emitted in the turbulences. Theagitation sensor can also be comprised of a sensor of the intensity ofthe sound waves partially reflected on the interface. The agitationsensor can also be comprised of a Doppler-effect sensor measuring theturbulences on the interface based on the partial reflection of soundwaves on this interface.

The control unit 145 operates as a means of closed-loop control of thedepth of the interface 170 between the freshwater and the seawater. Thecontrol unit 145 controls, the flow-rate of at least one pump 125pumping freshwater above said interface, according to said interfacedepth. When the interface depth goes over a certain threshold, or limitvalue, the control unit 145 increases the flow-rate of the pumped water.Conversely, when the interface depth is less than another threshold, orlimit value, the control unit 145 decreases the flow-rate of the pumpedwater. For preference, the flow-rate is a continuous function of thedepth, the control loop transfer function, in counter-reaction, iscalculated to avoid interface depth oscillations, in a way known topeople in the closed-loop control field.

For preference the control unit 145 takes account of the tide and isdesigned to maintain the interface below the level of at least onespring and, for preference, below the level of at least the spring withthe greatest flow-rate. In other words, by compensation, the depth isreplaced by the altitude of the interface with regard to a fixed pointand the control unit 145 controls the flow-rate of at least one pumpaccording to the sea level during the tide.

The control unit 145 thus constitutes a means of determining theagitation of the water outside at least one water source and inside theenvelop 110, and is designed to control the flow-rate of at least onepump according to the agitation of the water. In particular, when theturbulences exceed a pre-defined limit value, the control unit stops theoperation of the pump 125 or reduces it as described in a variant ofsteps 315 and 325 (see description of FIG. 3, below).

The control unit is also designed to stop the operation of the pump 125when the salinity measurement of the water pumped or to be pumped isgreater than a pre-defined value, which depends on the desalinationcapacities of the purification unit 140.

In a general way, the control unit 145 comprises a means 145 ofclosed-loop control designed to control the flow-rate of at least onepump 125 designed to pump freshwater above the interface between thefreshwater and the seawater, according to the status of said interface,especially its depth with respect to at least one or, if possible, allthe springs and/or turbulences of this interface in order to maintainthe depth of said interface below the opening of at least one saidfreshwater spring and, possibly, so that the highly saline watergenerated by these turbulences is not pumped.

As can be seen in FIG. 3, when it is started in normal operation, duringa step 305, the device measures the salinity of the water to be pumpedor, after a short period of pumping intended to ensure the sensor ispurged of salinity, the water pumped. Then, during a step 310, it isdetermined whether the salinity measured is greater than a firstpre-defined value. If it is, then during a step 315, the operation ofthe pump 125 is stopped and a restart command, either manual orautomatic, for example after a pre-defined length of time (for examplefive minutes), is waited for. Then you go back to step 305.

If, during the step 310, it is determined that the salinity measured isbelow the first pre-defined value, during a step 320, the magnitude ofthe turbulences at the interface between the seawater and the freshwateris determined. Then, during a step 325, it is determined whether themagnitude and/or speed of the turbulences is greater than a secondpre-defined value. If it is, then during step 315, the operation of thepump 125 is stopped and a restart command, either manual or automatic,for example after a pre-defined length of time (for example fiveminutes), is waited for. Then you go back to step 305.

If during a step 325 it is determined that the magnitude and/or speed ofthe turbulences is less than the second pre-defined value, then during astep 330, pumping is resumed, if it was stopped, or continued and thedepth of the interface between the seawater and the freshwater isdetermined.

In a variant, depending on the measurement of the turbulences of theinterface 170, the reduction in the flow-rate of the pumped water isinduced (for example, by the control unit 145) such that the depth ofthe interface 170 increases and the mixed water coming from theturbulences does not come in front of at least one, preferably all,freshwater spring opening(s). In this variant, if during a step 325, itis determined that the amplitude and/or speed of the turbulences isgreater than a second pre-defined value, the flow-rate of the pump 125is reduced and a command to resume the normal flow-rate of the pump iswaited for, which occurs when the interface is at a depth which is anincreasing function of the amplitude and/or speed of the turbulences.

During a step 335, the depth measurement is compensated for according tothe tide in order to obtain an absolute altitude value.

During a step 340, according to the depth of the interface and/or itsaltitude and according to the depth of at least the main freshwaterspring, or its altitude, and possibly the amplitude or speed of theturbulences, the water flow-rate that the pump 125 must provide isdetermined. The closed-loop control transfer function, which suppliesthis flow-rate according to the depths and/or altitudes, depends on theflow-rate of the springs in the envelop 110 and speeds of variation inthis flow-rate noted prior to the installation of the device or obtainedduring a learning phase or operational phase of the device. Forpreference, by means of the flow-rate of the pump 125, the interfacedepth is maintained below the opening of at least one freshwater springand, for preference, of all the freshwater springs.

During a step 345, the pump 125 is controlled so that it provides theflow-rate defined during the step 340. Then you go back to step 305.

1. Device for collecting water from at least one undersea freshwaterspring (105), characterized in that it comprises an envelop (110)surrounded by seawater isolating a quantity of water above at least onefreshwater spring, a sensor (150, 152, 155) of the status of aninterface (170) between the freshwater and the seawater, inside saidenvelop and a means (145) of closed-loop control designed to control theflow-rate of at least one pump (125) designed to pump freshwater abovesaid interface, according to said status of said interface, said meansof closed-loop control being designed to maintain the depth of saidinterface below the opening of at least one said freshwater spring. 2.Device according to claim 1, characterized in that the sensor (150, 152,155) of the status of said interface comprises a depth sensor designedto determine the depth of said interface and the means (145) ofclosed-loop control is designed to control the depth of said interfaceby controlling the flow-rate of at least one pump (125) according tosaid depth.
 3. Device according to claim 2, characterized in that thedepth sensor (150) comprises a sensor of the difference in level betweenthe surface of the freshwater inside the envelop (110) collecting thefreshwater and the surface of the seawater around said envelop. 4.Device according to claim 2, characterized in that the depth sensor(152) comprises an equilibrium depth sensor for a float possessing adensity half-way between the density of the freshwater and the densityof the seawater.
 5. Device according to claim 2, characterized in thatthe means (145) of closed-loop control is designed to maintain saidinterface below the level of at least the spring (105) with the greatestflow-rate.
 6. Device according to claim 1, characterized in that thesensor (150, 152, 155) of the status of said interface comprises a meansof determining the agitation of said interface and the means (145) ofclosed-loop control is designed to control the flow-rate of at least onepump (125) according to said agitation.
 7. Device according to claim 6,characterized in that the means (145) of closed-loop control is designedto reduce the flow-rate of at least one said pump when said interfacepresents turbulences greater than a pre-defined value.
 8. Deviceaccording to claim 6, characterized in that the means (155) ofdetermining the agitation of the water comprises a radar or sonardoppler-effect sensor.
 9. Device according to claim 6, characterized inthat the means (155) of determining the agitation of the water comprisesat least one water agitation sensor positioned in the water and a meansof measuring movements of each said sensor.
 10. Device according toclaim 6, characterized in that the means (155) of determining theagitation of the water comprises a means of determining a densitygradient of the water.
 11. Device according to claim 6, characterized inthat the means (155) of determining the agitation of the water comprisesa sensor of the difference in level between the surface of thefreshwater inside the envelop collecting the freshwater and the surfaceof the seawater around said envelop.
 12. Device according to claim 6,characterized in that the means (155) of determining the agitation ofthe water comprises an equilibrium depth sensor for a float possessing adensity half-way between the density of the freshwater and the densityof the seawater.
 13. Device according to claim 1, characterized in thatthe means (145) of closed-loop control is designed to control theflow-rate of at least one pump (125) according to the sea level duringthe tide.
 14. Device according to claim 1, characterized in that saidenvelop comprises a flexible partition surrounding the vertical above atleast one spring (105).
 15. Device according to claim 1, characterizedin that it comprises a means (151) of measuring the salinity of thewater pumped or to be pumped and a means of stopping pumping when saidsalinity is greater than a pre-defined value.
 16. Process for collectingwater from at least one undersea freshwater spring (105), characterizedin that it comprises a step (320, 330) of acquiring the status of aninterface (170) between the freshwater and the seawater, inside anenvelop surrounded by seawater isolating a quantity of water above atleast one freshwater spring and a step (340, 345) of closed-loop controlduring which the flow-rate of at least one pump (125) pumping freshwaterabove said interface is controlled according to said status of saidinterface, in order to maintain the depth of said interface below theopening of at least one said freshwater spring.
 17. Device according toclaim 3, characterized in that the depth sensor (152) comprises anequilibrium depth sensor for a float possessing a density half-waybetween the density of the freshwater and the density of the seawater.18. Device according to claim 3, characterized in that the means (145)of closed-loop control is designed to maintain said interface below thelevel of at least the spring (105) with the greatest flow-rate. 19.Device according to claim 4, characterized in that the means (145) ofclosed-loop control is designed to maintain said interface below thelevel of at least the spring (105) with the greatest flow-rate. 20.Device according to claim 7, characterized in that the means (155) ofdetermining the agitation of the water comprises a radar or sonardoppler-effect sensor.